This will likely be the time when you would like to have the 10.2 installer disks.
You should first double-check that you do have the correct install for the 10.2.8 updater on your CD. That file will be named: MacOSXUpdateCombo10.2.8.dmg
Note the word 'combo' in the middle. If that's not there, then you have the wrong download, and you need to get the correct one, from the link that I already provided.
This is how that's supposed to work: Double-click the .dmg, which will create a folder with a file named: MacOSXUpdateCombo10.2.8.pkg
When you then double-click that .pkg, that will launch the Installer utility, and you just follow the instructions to install the update.
If nothing about that works, and you actually do have the correct file, then your OS X system should be reinstalled using the option to Archive & Install - which means that you need the installer disks to fix this. There's not likely any other options that will help without reinstalling.